Magnesium fluoride is well-known as a coating material which imparts anti-reflectivity to optical articles and vision glazings, to increase the visible light transmissivity of same. Magnesium fluoride films may conveniently be deposited onto transparent substrates, either singularly or in combination with other metal and dielectric layers,.by conventional methods such as, for example, spray pyrolysis, chemical vapor deposition, vacuum evaporation, sputtering, and the like.
The deposition of magnesium fluoride by spray pyrolysis or chemical vapor deposition requires the preparation of a proper reactive precursor which is then directed onto the heated surface of the substrate to be coated. Spray pyrolysis utilizes a liquid reactive precursor which is sprayed onto the surface of the hot substrate whereat the precursor thermally decomposes and reacts to form a film. Chemical vapor deposition utilizes a gaseous reactive precursor which is delivered to the surface of the hot substrate whereat the precursor reacts to form the film. Conveniently, a continuous glass ribbon being produced by the float glass process possess sufficient heat to activate and react both spray pyrolysis and chemical vapor deposition precursors to form such films thereon.
U.S. Pat. No. 3,475,192 to Langley discloses a liquid reactive precursor which is coated onto a glass surface then fired to form a magnesium fluoride film. The precursor comprises magnesium fluoride dissolved in a mixture of an organic solvent and a film forming material, e.g., an oil.
U.S. Pat. No. 4,492,721 to Joosten et al. discloses a method of providing magnesium fluoride layers on substrates by the disproportionation of fluorine-containing organic magnesium compounds containing at least six fluorine atoms per magnesium atom. A reactive precursor, comprising a fluorine-containing compound, e.g., magnesium trifluoroacetate, magnesium trifluoroacetylacetonate, magnesium hexafluoroacetylacetonate, etc., and an organic solvent is contacted with a hot glass substrate utilizing either spray pyrolysis or chemical vapor deposition, depending upon the vaporization temperature of the fluorine-containing compound used. Such precursors, however, are generally difficult to work with due to their limited solubility in various solvents, and additionally produce coatings on glass having relatively limited anti-reflection properties.
It would be desirable to devise a process for depositing highly anti-reflective magnesium fluoride coatings on glass, utilizing reactive precursors which are easy to work with due to their good solubility properties.